OBJECTIVE Pathogenesis of AIDS involves substantial activation with concomitant destruction of lymph nodes and lymphoid tissues. Medical imaging approaches are useful for identifying tissue activation sites, using noninvasive methods for whole body analysis. Positron emission tomography imaging provides unique views of HIV or SIV infection and the pattern of host responses to these viruses. RESULTS Positron emmission tomography (PET) imaging using radioactively labeled deoxy glucoseis a method for visualizing highly activated tissues in the body. During chronic HIV or SIV infection, lymphoid tissues are activated to a very high extent and are easily visualized by this method. Studies of early times after experimental virus inoculation in macaques and studies of patients chronically infected with HIV-1, showed a definite pattern of tissue activation in relation to disease progression. In a manner very similar to progressing lymphoma, the virus infections activated lymphoid tissues in the upper torso during early stages of infection. As disease progressed, the pattern of activation extended down the torso and eventually involved iliac and inguinal lymph node groups. The final stages of disease involved activation of gut-associated lymphoid tissues. HIV-infected patients classified as long term survivors had patterns of tissue activation restricted to upper torso in good ag reement with their excellent clinical status. PET imaging provides unique views of AIDS pathogenesis and may become a novel tool for evaluating therapeutic efficacy. FUTURE DIRECTIONS Experiments are continuing to explore the application of PET imaging for understanding the earliest events of acute infection in macaques. Additionally, we received NIH funding to pursue our characterization of PET images from HIV-infected patients and that work is underway. KEY WORDS AIDS, therapy, progression, lymph node, pathology, imaging FUNDING NIH R01 AI24591 and NIH R01 AI33237